Pham, a critical care nurse at Texas Health Presbyterian Hospital, is one of at least fifty people who treated Ebola patient Thomas Eric Duncan, who contracted the disease in Liberia and then traveled to the U.S., where he first started showing symptoms. He died last Wednesday.

Pham is reportedly in stable condition. It's possible that there may be additional cases among the health care workers who treated Duncan.

Dr. Thomas Frieden, the director of the Centers for Disease Control and Prevention, said that Pham is in stable condition, but may not be the only person that became infected while treating Duncan.

"We have to consider the possibility that there are other cases," Frieden said Monday.

"Unfortunately [I] would not be surprised if we did see additional cases among the health care workers who treated the index patient," he said referring to Duncan.

He said the other members of the team that treated Duncan at Texas Health Presbyterian Hospital Dallas are being closely monitored, and health officials are tracking Pham's contacts since her symptoms appeared. Freiden said there appeared to be only one person who was in contact with Pham once she became infectious.

"Stopping Ebola is hard," Frieden said.

He said because of the infection of a health care worker who was following CDC guidelines, "We have to rethink the way that we address Ebola infection control."

Pham's home was sealed off, and has been inspected by cleaned by the Centers for Disease Control. Officials are trying to trace contacts and find anyone who may have been exposed to Ebola in the area.

Authorities are calling Pham "a hero." They are also keeping an eye on the nurse's dog, which remains in her apartment and is being cared for while Pham is being treated.